Non-alcoholic fatty liver disease (NAFLD), defined by triacylglycerol (TAG) accumulation, is a prevalent disorder that is involved in the etiology of numerous metabolic diseases including obesity, diabetes and cardiovascular disease. Despite the importance of TAG metabolism and the advancements into our understanding of the TAG synthetic pathway, the mechanisms regulating hepatic TAG hydrolysis and their effects on disease etiology are unknown. The objective of this application is to define the role of adipose TAG lipase (ATGL) in hepatic lipid metabolism and signaling, the etiology of insulin resistance and in mediating diet-specific effects. The hypothesis of the proposed studies is that ATGL is a prominent hepatic lipase that elicits wide-ranging effects on energy metabolism by altering FA channeling and signaling. We base this hypothesis on Preliminary Studies from our laboratory showing that ATGL alters partitioning of hydrolyzed FA between anabolic and catabolic pathways, regulates PPAR-1 activity, uncouples insulin resistance from NAFLD and selectively hydrolyzes TAG. The rationale for the proposed research is that identifying the role of ATGL in mediating hepatic TAG metabolism will provide significant insight into the etiology of NAFLD and related comorbidities, thereby, advancing the possibilities for development of nutritional or pharmaceutical therapies. The hypothesis will be tested using three specific aims: 1) to characterize the ATGL and PPAR-1 signaling axis and its role in hepatic energy metabolism, 2) to define the role of hepatic ATGL in regulating hepatic and whole-body insulin resistance and 3) to characterize the effect of hepatic ATGL on hydrolyzing distinct TAG pools and mediating subsequent FA channeling and signaling. Under the first aim, ATGL gain or loss-of-function studies in mouse models will define the relationship between ATGL and PPAR-a. Additionally, in vitro studies will define the mechanisms through which ATGL regulates PPAR-1 activity. The second aim will employ both in vitro and in vivo methodologies along with stable isotope techniques to characterize how ATGL uncouples hepatic TAG accumulation from insulin resistance and its role in the formation of lipid signaling molecules. The third aim will define how hepatic ATGL regulates the signaling and channeling of TAG-FA derived from different substrates and mediates diet-specific effects on metabolism. These studies are innovative because they approach NAFLD and its comorbidities from the understudied and often ignored pathway of TAG hydrolysis. Understanding how hepatic TAG hydrolysis is regulated is significant because it not only impacts TAG accumulation, but also dictates the metabolic fate and signaling properties of the hydrolyzed FA that can ultimately influence a multitude of biological processes.